This invention relates to interbody spinal fusion implants and surgical procedures for implanting load bearing devices in intervertebral disc spaces after removal of damaged or diseased discs, with the implanted devices being securely connected to the adjacent vertebrae to stabilize the vertebrae while providing for proper transmission of loads therebetween, and being hollow to receive bone graft for fusion of the vertebrae.
More particularly, the present invention relates to interbody spinal fusion implants and a procedure for surgically implanting a device of novel design that 1) has a thickness selected to restore and maintain proper spacing between the end plates of two adjacent vertebrae of the spine, 2) has its top and bottom surfaces inclined to define a wedge shape when viewed in side elevation if such inclination is needed to restore and maintain normal spinal lordosis by restoring and maintaining the normal anatomic angular relationship of the vertebrae, 3) is rigidly connected to each of the adjacent vertebrae by large screws that angle upwardly and downwardly from an anterior or side region of the implant and that thread through the vertebral end plates and into the soft cancellous bone in the central regions of the adjacent vertebrae to prevent relative movement of the adjacent vertebrae, 4) defines a hollow central region for receiving bone graft material for fusing the vertebrae, and 5) provides annular load-carrying portions of the implant that surround the central region and function to transmit loads directly, in a physiologic manner, from the hard cortical periphery of one vertebral end plate to the hard cortical periphery of the adjacent vertebral end plate.
The vertebrae of the human spine are arranged in a columnar array one atop another, with each horizontally extending space between adjacent pairs of the vertebrae being provided with a separate intervertebral disc that transmits between adjacent vertebrae such forces as are imposed on and carried by the spine, while also functioning to cushion and permit a limited amount of relative movement to take place between the adjacent vertebrae. In a healthy spine, 1) each vertebra is composed of a hard outer ring of cortical bone, and a softer center of cancellous bone; 2) each intervertebral disc is composed of a tough outer ring of fibrous material, and a softer center of jelly-like material; and, 3) the physiologic manner in which loads are carried by the spine makes use of the hard fibrous annuli of the discs to transmit force between the hard cortical periphery of the vertebrae.
The cervical and lumbar areas of the human spine are, in a healthy state, lordotic such that they curve convexly forward. Normal lordosis results, at least in significant measure, from the normal wedge-shaped nature of the spaces between adjacent pairs of the cervical and lumbar vertebrae, and the normal wedge-shaped nature of the intervertebral discs that fill these spaces. If disc damage or degeneration occurs, lordosis tends to be lost, at least in part, because the wedge-shaped character of the discs diminishes and may become lostxe2x80x94hence, the wedge-shaped character of the spaces between adjacent cervical and lumbar vertebrae is likewise caused to diminish, and normal spinal curvature is altered as a result. Moreover, if normal disc thickness diminishes, as is common in the presence of disc damage or disease, the normal spacing between the vertebrae is thereby caused to diminish, causing the height of the spinal column to be undesirably diminished. Loss of lordosis and loss of proper vertebral spacing disturb the overall mechanics of the spine, often causing cascading degenerative changes.
Disc degeneration and the spinal changes that result can bring pain. An accepted treatment for back pain caused by a degenerative disc is to remove the disc and fuse the adjacent vertebrae in a manner that maintains suitable spacing of the vertebrae while preventing the vertebrae from moving relative to each other, for example by filling the intervertebral space where the disc was removed with bone graft that will enable the adjacent vertebrae to grow together and become one solid piece of bone.
The main front (i.e., anterior) portions of adjacent vertebrae between which an intervertebral disc normally resides are referred to by the term xe2x80x9cvertebral bodies.xe2x80x9d The anterior space between adjacent vertebral bodies where a disc normally resides is referred to by the term xe2x80x9cintervertebral disc space.xe2x80x9d When bone graft material that has been inserted into the intervertebral disc space fuses the vertebral bodies of adjacent vertebrae, the process is referred to as xe2x80x9canterior interbody fusion.xe2x80x9d
Anterior interbody fusion may require weeks, sometimes months to achieve a desirable result, and is likely to achieve unsatisfactory results if relative movement takes place between the adjacent vertebrae while fusion is underway. If relative movement of adjacent vertebral bodies takes place while fusion is underway, this will, as a minimum, slow the rate of fusion, and can prevent acceptable fusion results from being achieved.
Also, if relative movement takes place while fusion is underway, significant continued back pain after surgery may result. To stabilize the adjacent vertebrae and prevent relative movement from taking place, one traditional approach has been to place the patient in a body cast, a procedure that still is used in many instances. To avoid using a body cast and to improve the prospects for achieving satisfactory fusion by more directly immobilizing adjacent vertebrae, some surgeons have used steel rods and other devices that are secured to the posterior of the vertebrae. A significant drawback associated with the use of such posterior appliances is the need for posterior surgery to put them in place.
Operating on the spine from a posterior approach disrupts posterior muscles causing permanent muscle dysfunction, referred to as xe2x80x9cfusion diseasexe2x80x9dxe2x80x94hence, posterior back surgery is to be avoided unless essential. Other drawbacks accompanying the use of such posterior immobilizers include the need for a second surgery to remove these temporary appliances once satisfactory fusion has been achieved, and the discomfort, inconvenience and danger that can result from having to live with this hardware for weeks or months following the original surgery.
While a variety of anterior interbody fusion techniques and implant devices have been developed for use in the cervical region of the spine where the vertebrae are smaller and the loads that must be transmitted between adjacent vertebrae are smaller than in the lumbar region of the spine, it has proven more elusive to achieve consistently good anterior interbody fusion results in the lumbar region of the spine without having to employ body casts or temporary installations of posterior immobilizers.
An approach that has been used with greater success to achieve anterior interbody fusion in the lumbar region of the spine than in the cervical region of the spine has been to install, within an intervertebral space where a degenerative disc has been removed, a pair of so-called xe2x80x9cthreaded fusion cages.xe2x80x9d Threaded fusion cages are of generally cylindrical form and are available in a variety of diameters. If threaded fusion cages are to be installed in an intervertebral space from which a degenerated disc has been removed, cages are selected that have sufficiently large diameters to enable their threaded exteriors to xe2x80x9cthreadxe2x80x9d or xe2x80x9cbitexe2x80x9d into underportions of the vertebral body of the upper vertebra, and into upper portions of the vertebral body of the lower vertebra as each of the cages are xe2x80x9cthreadedxe2x80x9d into the intervertebral space.
After the threaded fusion cages are installed, their hollow interiors are filled with bone graft material so that bone growth will take place within and about the threaded fusion cages while the cages serve to maintain proper spacing between adjacent vertebrae.
While bone growth is taking place, spinal loads are transmitted between the adjacent vertebrae principally by the fusion cages inasmuch as the bone graft material is not capable of participating to a significant degree in carrying compressive force. If bone growth progresses satisfactorily to provide good fusion results, the one-piece bone structure that is formed from the adjacent vertebrae relies principally upon its own integrity and strength to carry spinal loads.
In a number of instances, threaded fusion cages have failed to function in the desired manner to provide needed back pain relief and/or to maintain adequate stability to enable proper fusion results to be achieved. In some instances, threaded fusion cages not only have failed to provide one or both of these basic objectives, but also have been found to cause nerve damage or to present other disadvantages and drawbacks. When unsatisfactory results have been obtained even though accepted procedures for installing threaded fusion cages have been followed, it is believed that one or more of the following explanations may apply:
1) Threaded fusion cages engage very little of the hard peripheral ring of cortical bone of each of two adjacent vertebrae; rather, the fusion cages mainly reside between vertebral regions of soft cancellous bone. Thus, threaded fusion cages rely significantly if not principally on the soft cancellous bone centers of adjacent vertebral bodies to transmit loads between adjacent vertebrae rather than on the hard cortical peripheries of the vertebral bodies which are vastly better suited for this purpose. This reliance on soft rather than hard bone tissue to transmit loads along the spine violates the most basic of orthopedic biomechanical principles, is opposite to the natural state wherein the tough fibrous periphery of an intervertebral disc transmits loads between the hard peripheral bone areas of adjacent vertebral bodies, and unquestionably accounts for some of the complications that are seen when threaded fusion cages subside into vertebral bodies and fusion fails to occur. Furthermore, because the generally cylindrical outer surfaces of the fusion cages engage the end plates of adjacent vertebral bodies along relatively small, relatively narrow areas of engagement, they do not apply the loads carried by the spine to large surface areas of the end plates of the vertebral bodies, but rather tend to apply these loads in a concentrated manner along the narrow regions where they engage the vertebral body end plates, which can cause one or both of the fusion cages installed between two adjacent vertebrae to puncture one of both of the vertebral end plates and subside into the vertebrae.
2) Threaded fusion cages are not rigidly fixed to the adjacent vertebral bodies they engage. Despite the tendency of their threaded exteriors to xe2x80x9cbitexe2x80x9d into the end plates of adjacent vertebrae, the generally cylindrical character of the fusion cages can permit relative movement of the vertebrae to take placexe2x80x94much as would occur if two elongate metallic xe2x80x9crollersxe2x80x9d were inserted between two stacked blocks of wood. The roller-like character of the cages can, and in some instances does, fail to immobilize the adjacent vertebrae, and the unwanted relative movement that may result is disruptive to fusion. Moreover, because the generally cylindrical fusion cages are not rigidly, mechanically attached to the adjacent vertebrae, threaded fusion cages cannot be used where there is preexisting instability of the spine. If spinal instability exists for any reason, the use of threaded fusion cages risks undesirable results.
3) Threaded fusion cages are not anatomically shaped to conform to the intervertebral spaces in which they are installed. Rather, they are of substantially uniform diameter along their lengths, with their diameters being chosen to correspond to the vertical height of the interbody space they are to occupy. What this means is that the wider (or taller) the intervertebral space into which fusion cages are to be threaded, the larger the diameter of cages that must be selected, otherwise their threaded outer surfaces will fail to xe2x80x9cbitexe2x80x9d into the end plates of each of the adjacent vertebral bodies. In some instances, the pair of threaded fusion cages that are installed side by side in a wide intervertebral space are of a diameter sufficiently large to crowd nerves and/or to put other tissue at risk. Nerve injury has been observed to occur with greater frequency as fusion cages of increasingly greater diameter are utilized. The replacement of a damaged or diseased disc that is especially thick or plump may present a challenge that cannot be suitably addressed through the use of large diameter threaded fusion cages in view of the unacceptable nerve crowding that would result.
4) Threaded fusion cages are not designed to restore and maintain the normal curve of the lumbar spine, known as lordosis. Normal intervertebral discs have something of a xe2x80x9ctapered wedgexe2x80x9d shape, with the vertebral endplates of adjacent vertebrae defining surfaces that do not normally extend in parallel planes. When discs degenerate and collapse, they cause the spine to lose some of its normal curvature, which disturbs normal biomechanical function, and can lead to further deterioration. Threaded fusion cages do nothing at all to restore and maintain proper lordosis. Rather, threaded fusion cages seek to fix the configuration of the spine with the end plates of adjacent vertebral bodies extending in substantially parallel planes.
5) Nor do threaded fusion cages necessarily function to reestablish proper spacing between adjacent vertebrae. When a collapsed or degenerated disc is removed and threaded fusion cages are inserted to maintain the disc spacing that then exists, the intervertebral disc space may be less than it should be to accommodate the volume or capacity of the spine and perhaps should be widened to more closely reestablish normal physiologyxe2x80x94but widening the intervertebral space will require that threaded fusion cages of greater diameter be installed, and the problems that may result with the use of larger diameter cages may be significantly more devastating than will result if the narrowed intervertebral space is maintained, so smaller diameter fusion cages may be selected for use, and the problem of diminished intervertebral disc spacing may be largely ignored.
The present invention addresses the foregoing and other needs and drawbacks of the prior art by providing an anterior lumbar interbody fusion method and device that can be utilized to restore and maintain proper lumbar spine curvature and proper intervertebral disc spacing, and that transmits spinal loads in a proper physiologic manner through the intervertebral space where a regenerative disc has been removed from the periphery of hard cortical bone of one vertebra directly to the periphery of hard cortical bone of the adjacent vertebra while being rigidly connected to both of the adjacent vertebral bodies to thereby positively prevent relative movement from taking place therebetween, and while providing a hollow central region where bone graft is inserted so that an optimal environment is provided anterior lumbar interbody fusion to take place in a minimum of time and with good results.
In accordance with the invention, there is provided a one-piece implant that is shaped roughly the same as a normal disc.
According to another aspect of the invention, there are provided implants that are of different left-to-right and front-to-rear sizes for use between vertebral bodies of different sizes.
According to another aspect of the invention, there are provided implants that differ in bottom-to- top thickness.
According to another aspect of the invention, there is provided means for varying the degrees of wedge inclination between the planes of the top and bottom surfaces of the implants.
According to another aspect of the invention, the implant is rigidly coupled to the adjacent vertebral bodies through the use of relatively large screws of a proven orthopedic design.
According to another aspect of the invention, the implant defines a pair of inclined screw passages that are accessible either anteriorally or laterally for guiding threaded screws through the end plates of adjacent vertebral bodies to assist in ensuring that the screws are installed at proper angles of inclination to form effective connections with the adjacent vertebrae.
According to another aspect of the invention, there is provided a countersunk shoulder at the entryway of the screw passages for reception of the screw heads.
According to another aspect of the invention, the implant is generally annular in shape comprising a peripheral ring of rigid material that surrounds a hollow central area. The rigid periphery fits directly between the hard cortical peripheral rings of the adjacent peripheral bodies, but does nothing to crowd adjacent nerves or other tissue.
According to another aspect of the invention, the implant includes a relatively large open central region that can accommodate a sizable volume of bone graft material, wherein the rigid periphery of the implant serves as a protective fence that guards the region where bone growth takes place as fusion is underway.
One advantage of the invention, is provided by the more natural shape of the implant and the fact that it preferably is a one-piece, integral structure. The implant is therefore easier to insert and to properly position in an intervertebral space than two fusion cages which typically require the use of rotary tools to drill selected locations and to rotate the cages to thread them into the intervertebral space under flouroscopic guidance and with an exceedingly small margin for error. Accurate cage placement is quite essential; perfecting the technique to effect proper cage placement is difficult; and, the need for constant flouroscopic imaging exposes the patient to a high dose of radiation. By contrast, implants embodying the preferred practice of the present invention are easier to place, more forgiving of technique, and require only intermittent X-ray images to effect satisfactory placement.
Another advantage of the invention is that the implant can be used in many situations where there is preexisting instability of the spine inasmuch as the preferred form of implant takes much the same shape as a healthy disc and is rigidly connected by screws to the adjacent vertebral bodies.
Another advantage of the present invention is the provision of an implant that can be selected of proper size and thickness to appropriately maintain proper intervertebral spacing, with its hard peripheral regions being properly positioned to transmit spinal loads directly between the hard cortical peripheries of the adjacent vertebral bodies. Moreover, an implant can be selected to conform the intervertebral space to an appropriate wedge shape and to maintain proper spacing between the adjacent vertebral bodies so that proper spinal lordosis is reestablished and/or maintained, and so that the height of the spinal column is not inappropriately shortened.
Another advantage of the invention is that use of the large screws reliably immobilizes the vertebrae in relation to each other to provide an optimum environment within which good fusion results are likely to be received. Also, by eliminating relative movement of the vertebrae to be fused, instances of continuing back pain following surgery are significantly diminished, thereby enhancing the comfort of the patient especially during the higher risk time while the fusion process is underway.
Another advantage of the invention is that when screws are installed and tightened in place to seat their enlarged heads against the countersunk shoulders provided at the entryways of these passages, reliably rigid connections are established between the implant and the adjacent vertebral bodies. Providing countersunk or recessed openings to receive the heads of the screws is of particular importance when the screws are installed anteriorally (as opposed to laterally, from the left and/or right) to prevent the screw heads from projecting into engagement with adjacent vessels (e.g., the inferior vena cava and the aorta).
Another advantage of the invention is that the need to replace an especially thick or plump damaged or diseased disc does not bring with it nerve crowding problems such as are encountered where a pair of large diameter threaded screw cages are inserted into a thick intervertebral space.
Another advantage of the invention is the ability to utilize a sizable quantity of bone graft material and the protective central positioning of a majority of the bone graft material between the soft cancellous regions of the adjacent vertebral bodies provides a truly optimal environment for rapid and proper bone growthxe2x80x94so that good fusion results tend to be obtained within a minimum of time.
In the detailed discussion that follows, other features and advantages also will become apparent. For example, two implant embodiments are disclosed, one that is intended to be installed from the front of the spine (i.e., anteriorally), and one that is designed to be inserted from the side of the spine (i.e., laterally). While the anterior version of the implant preferably provides an access opening through the anterior side wall of the implant for inserting bone graft, the lateral version of the implant preferably provides at least one access opening through at least one of the lateral side walls of the implant for inserting bone graft.
Other improved features are contemplated, including a more complex form of the implant that provides a capability to telescope to adjust its thickness and perhaps also its wedge-shaped character as it is being inserted and positioned in an intervertebral spacexe2x80x94features that are likely to render the implant even more suitable to install utilizing techniques of minimally invasive surgery (surgery that typically utilizes a video endoscope).
Also contemplated by the invention is a preferred method for replacing a degenerated intervertebral disc and fusing the adjacent vertebrae utilizing a novel interbody implant device of the type described above that preferably is sized, configured, positioned and secured in the manner described above to provide the advantages and features described abovexe2x80x94advantages and features that permit the implant to be used with improved success and safety to achieve good results more consistently than presently is the case with the use of threaded fusion cages or other forms of implants and surgical techniques.